The present invention relates to a permanent magnet DC machine, and more particularly relates to a permanent magnet DC machine having a stator provided with main magnetic poles of anisotropic permanent magnets and auxiliary magnetic poles having higher reversible permeability than that of the permanent magnets and arranged side by side with the main magnetic poles.
Conventionally, a permanent magnet DC machine including a stator or the like, is provided with auxiliary magnetic poles of soft iron or the like on the magnetism-increasing side as a countermeasure against demagnetization due to an armature reaction as taught in Japanese Patent Examined Publication No. 35721/73. That is, there have been proposed an arrangement of a permanent magnet DC machine in which main magnetic poles of anisotropic permanent magnets having their opposite ends cut parallelly to center lines extended radially from the center of a rotor and auxiliary magnetic poles of soft iron or the like are provided so as to be adjacent to one another through circumferential air gaps, respectively; and another arrangement of a permanent magnet DC machine in which main magnetic poles and auxiliary magnetic poles are provided so as to be adjacent to one another with their facing surfaces being in close contact with each other.
It has been well known that the magnetic flux of the anisotropic permanent magnet passes through a rotor in the radial direction thereof. Accordingly, in the case where the auxiliary magnetic pole is arranged adjacent to the main magnetic pole of the anisotropic permanent magnet so that they are in close contact with each other at their entire facing surfaces in the circumferential direction of a stator (that is, the rotational direction of a rotor), a leakage magnetic path is formed between those magnetic poles. Therefore, in such a permanent magnet DC machine in which the entire facing end surfaces of the main and auxiliary magnetic pole are in close contact with each other, the effective magnetic flux is reduced to thereby reduce the rotary torque of the machine.
On the other hand, in such a permanent magnet DC machine in which an auxiliary magnetic pole is arranged adjacent to a main magnetic pole of an anisotropic permanent magnet through an air gap in the circumferential direction of the stator (the rotational direction of the rotor), leakage magnetic flux is reduced. However, there is a problem in that since an air gap is formed between each pair of main and auxiliary magnetic poles over the full length of the stator, the magnetic flux distribution waveform viewed from the armature side of the rotor drops sharply between the main and auxiliary magnetic poles at the time of high load, so that smooth driving becomes difficult, the output becomes lower, and magnetic sound and/or mechanical vibrations are generated. There is a further problem in that the formation of an air gap between each pair of main and auxiliary magnetic poles makes it difficult to place the main and auxiliary magnetic poles in position, resulting in poor assembling property.